Automotive crankcase lubricants generally contain appreciable amounts of overbased metal compounds, particularly overbased alkaline earth metal or magnesium petroleum or alkyl benzene sulphonates. These overbased metal compounds provide effective rust inhibition by neutralizing corrosion acidic combustion products from the fuel, such as hydrochloric and hydrobromic acid from the lead scavengers normally present and low molecular weight organic acids from the oxidation of the fuel itself. In the absence of overbased additives, as in ashless oils, or when such additives are present in reduced amounts, as in "low-ash" oils, rusting becomes a serious problem.
Various commonly known ashless rust-inhibitors have been tried in ashless automotive crankcase oils with little success. However, we have now found a combination of additives providing effective rust-inhibition in ashless oils as well as in oils which contain ash-producing components.